This invention concerns crystals for nonlinear optical devices, in particular for the generation of coherent ultraviolet light by mixing or frequency doubling of longer wavelength light.
Crystals for nonlinear optical applications are normally found among those materials which lack a center of symmetry and are at least weakly piezoelectric. Numerous examples of nonlinear optical crystals have been discovered in recent years, most of which were already known to be piezoelectric. It was quickly found that tremendous enhancement of the second harmonic generation intensity was obtained under phase-matched conditions, i.e., when the index of refraction of the primary beam equals that of the doubled frequency vibrating at right angles to the direction of the primary beam (1). The wide range of crystals now available allows phase matching over a wide range of wavelengths, not including, however, the ultraviolet below 2300 A. The reason for this is that most materials strongly absorb light of these lower wavelengths, which eliminates or greatly reduces UV intensity. A second problem is the phase matching requirement. All materials which are transparent in the visible eventually reach an absorption edge in the UV. As this absorption edge is approached the indices of refraction increase at an increasingly rapid rate, effectively eliminating any possibility of phase matching. At present, two crystals which are used for phase matched frequency doubling into the UV are ammonium dihydrogen phosphate (ADP) which will room-temperature phase match to about 2624 A (2), and the recently reported lithium formate monohydrate, which is limited by ultraviolet absorption to 2360 A and above (3). A crystal which will phase match somewhat farther into the UV is most desired.